When an exterior doorway of a building is used as a loading dock for vehicles, such as trucks and tractor/trailers, the perimeter of the doorway often includes a dock seal. Dock seals close off gaps that would otherwise exist between the exterior face of the building and the back end of the trailer. Dock seals allow cargo from the rear of the trailer to be loaded or unloaded while dockworkers and the cargo are protected from the weather. Usually two side seals run vertically along the lateral edges of the doorway, and a top or head seal runs horizontally along the doorway's upper edge; however, additional seals can also be used.
A typical dock seal comprises a resiliently compressible foam core supported by a rigid backer, such as a wood plank or a formed metal plate. The foam core and backer are normally encased within a fabric outer cover. Sealing is provided by backing the trailer up against the seal so that the seal compressively conforms to the rear shape of the trailer. The foam core provides the necessary compliance and resilience to repeatedly conform to the shape of various trailers; the outer cover protects the foam core from dirt, water and wear; and the backer provides solid structure for mounting the seal to the wall and for supporting the foam core so that the foam core does not twist and roll within the fabric cover.
Due to the trailer's wheel suspension, adding or removing cargo and/or driving a forklift on and off the trailer bed can cause the rear of the trailer to repeatedly rise and lower a few inches. Although the incidental movement can be a problem, most dock seals are sufficiently wear resistant to tolerate such movement.
A more serious problem, however, can occur after a tractor backs its trailer up against the dock seal, and the trailer is subsequently unhitched from the tractor while the trailer is still up against the seal. The front end of the unhitched trailer might then be set back down on the trailer's landing gear or temporarily rehitched onto a special tractor (yard jockey or yard mule) designated specifically for shuffling trailers around the loading dock area. Hitching and unhitching the front end of the trailer can cause the entire trailer to tilt about its rear wheels. The resulting seesaw action produces substantial up and down movement at the rear end of the trailer, which can cut and abrade the dock seal.
Moreover, when the front end of the trailer is raised, which tilts the rear end of the trailer downward, the upper rear edge of the trailer can dig deeply into the dock's head seal. When the front end of the trailer is subsequently lowered, the trailer's rear edge can pry the head seal upward.
In some cases, the trailer's rear edge digs into the seal so deeply that the edge catches the seal's backer and pries the head seal off the wall. This particularly tends to happen with relatively thick backers that are made intentionally thick to provide the foam core with ample support. If the backer is too thin, however, or omitted entirely in order to prevent the trailer's rear edge from catching the backer, the foam core may tend to roll and twist within the outer fabric cover. Thus, it can be difficult to design a backer with a thickness that addresses both problems.